Estrogens regulate the growth, differentiation, and functioning of diverse target tissues, both within and outside of the reproductive system. Most of the actions of estrogens appear to be exerted via the estrogen receptor (ER) of target cells, an intracellular receptor that is a member of a large superfamily of proteins that function as ligand-activated transcription factors, regulating the synthesis of specific RNAs and proteins. To understand how the ER discriminates between estrogen ligands, which activate the ER, and antiestrogen ligands, which fail to effectively activate the ER, we have generated and analyzed human estrogen receptors with mutations in the ER hormone binding domain. These studies provide evidence for the promoter-specific and cell-specific actions of the estrogen-occupied and antiestrogen-occupied and antiestrogen-occupied ER, highlight a regional dissociation of the hormone binding and transcription activation functions in domain E of the receptor, and indicate that some of the contact sites of estrogens and antiestrogens in the ER are likely different. In addition, multiple interactions among different cellular signaling pathways are involved in the regulation of gene expression and cell proliferation by the ER. In several cell types, protein kinase activators and some growth factors enhance the transcriptional activity of the ER. Cyclic AMP also alters the agonist/antagonist balance of some antiestrogens. Estrogens and, to a lesser extent, antiestrogens, as well as protein kinase activators and growth factors increase phosphorylation of the ER and possibly other proteins involved in the ER-specific response pathway, suggesting that changes in cellular phosphorylation state will be important in determining the biological activity of the ER and the effectiveness of antiestrogens as estrogen antagonists. The ER also has important interrelationships with the progesterone receptor (PR) system in modulation of biological responses. Liganded PR-A and PR-B can each suppress estradiol-stimulated ER activity, with the magnitude of repression dependent on the PR isoform, progestin ligand, promoter, and cell type. These findings underscore the mounting evidence for the importance of interactions between members of the steroid hormone receptor family.